The transduction of light-energy into a receptor potential in the rod photoreceptor of the vertebrate retina commences with adsorption of the light by the membrane bound visual pigment rhodopsin. Upon absorption of light, rhodopsin undergoes a sequence of conformational changes and these changes lead, through an unknown mechanism, to the change in plasma membrane ionic permeability which underlies the rod photoresponse. The object of the project proposed herein is to understand the mechanisms which couple the conformational change of rhodopsin to the change in plasma membrane permeability. In particular, the transduction mechanism involves a modification of the levels of free Ca ion in the photoreceptor cell. Through the use of ion specific membrane electrodes the characteristics of these concentration changes will be investigated. The molecular mechanism through which the intact rod cell achieves such charges will also be studied. In addition to charges in Ca ions, phototransduction involves the activation of several membrane bound enzymatic processes. These will be studied by measuring the enzymatic activity in a system that permits complete experimental control of the transmembrane gradients of ions and voltage.